12 - Genetic Drift
Population bottleneck steps
- Ecological events may reduce population sizes dramatically e.g. earthquakes, floods, fires (disasters that are unselective) - Small surviving populations are unlikely to be representative of the original population. - By chance alleles, may be overrepresented among survivors, while some may be eliminated completely.
In small populations:
- Individuals make a relatively large contribution to the gene pool. - Evolution can occur rapidly. - Many changes are non-adaptive. - Many changes are due to chance events.
In large populations:
- Individuals make only a small contribution to the gene pool. - Evolution occurs slowly. - Most changes are adaptive. - Natural selection is the main driving force.
Steps of the Founder Effect
- Occasionally a small group of individuals may migrate away or become isolated from a population. - The 'founding' population is only made up of a small number of individuals. Inbreeding may be a problem if individuals are closely related. - It may have a non-representing sample of alleles from the parent population. - The colonizing population may evolve quite differently from the original population, especially if the environment is different.
Example of the Founder Effect: The Fugates of Kentucky
- Small founding population in mountain communities; 2 of the founders were carriers of a recessive allele for blue skin.
Population bottlenecks
Anything that creates a sudden drop in population size (e.g. wars, natural disasters or migration), or prevents individuals from breeding, reduces mating possibilities and can cause a genetic bottleneck. This can greatly reduce genetic variation within the population.
The effects of genetic drift
Can be amplified by differences in the number of children raised by couples, or individuals dying prematurely; can result in: - traits being lost from small populations. - unusual traits, not commonly found in the parent population, and that are often non-adaptive, becoming established.
Effects of inbreeding
Can result in non-adaptive changes occurring. Whereas it can amplify desirable traits, it can also greatly increase the risk of unusual, often harmful traits being expressed.
What is a possible way in which various types of organism can be carried to an offshore island?
Continental drift and the movement of the tectonic plates.
Natural selection
Determines which mutations remain in a gene pool, and which ones are eliminated.
Genetic diversity
Genetic differences between individuals within a population (in terms of alleles)
The Founder Effect
Genetic drift that occurs when a few individuals become isolated from a larger population and form a new population whose gene pool composition is not reflective of that of the original population.
What can populations get isolated by?
Geographical barriers such as mountain ranges, deserts, swamps, rivers.
Allele frequency
How often a particular allele occurs within a population.
Example of Bottleneck Effect: Northern Elephant Seals
Hunted close to extinction, individuals on islands survived. Reduced genetic diversity compared with southern elephant seals.
Consanguinity
Kinship; relationship by birth.
Natural selection
Natural selection is the scientific theory proposed by Charles Darwin and Alfred Wallace that organisms best adapted to their environment tend to survive and out-multiply those that are less well adapted.
Natural selection vs. Genetic drift
Natural selection is to do with the biologically fittest organism, the one which will survive to reproduce. Genetic drift is completely random whether it be due to a catastrophic event (bottleneck effect) or isolation (founder effect), it is also to do with a change in allele frequency.
Example of the Founder Effect: Pitcairn Island
Pitcairn Island, in the Pacific, was colonised by the mutineers from HMS Bounty in 1790. Today the population of the island is around 44 people.
In-breeding
Reduced mating opportunities often lead to an increase in in-breeding, or consanguinity; in-breeding results in reduced genetic diversity in the population's gene pool.
Example of the Founder Effect: Amish
The Old Order Amish community of Lancaster County in Pennsylvania, USA is a fundamentalist religious sect who do not marry out or use modern technology. The Amish community numbers around 18 000 people and shares only eight family names. Within the community there is an above average incidence of inherited illnesses including a rare form of microcephally (small brain), albinism, dwarfism, cretinism, webbed fingers and limb girdle muscular dystrophy.
What creates changes in gene pools?
The allele frequency in a population's gene pool can be affected by evolutionary mechanisms, such as natural selection, or by chance occurrences, such as founder effect and random genetic drift.
Speciation
The formation of new and distinct species in the course of evolution.
Example of Genetic Drift
The frequency of a particular trait could, for no obvious reason, drift from 2% in generation 1, to 11% in generation 2, to 5% in generation 3 etc.
Genetic Drift
The random fluctuation of allele frequencies in a population from one generation to the next, often a consequence of a genetic bottleneck i.e. it results from inbreeding brought about by the limited mating possibilities in a small community.
Gene pool
The sum total of genes, with all their variations, possessed by a particular species at a particular time.
Example of Bottleneck Effect: Cheetahs
•10,000 years ago all but 1 species had died out; severely threatened. Very low genetic diversity resulting in poor sperm quality among males meant that females forced to breed with close relatives. Inbreeding generally decreases the fitness of a population (an inbreeding depression.)
